Well I only have 20 frets and about 5 of those are pretty much unused except for checking intonation and playing scales. I use the 12th fret for bridge placement and just checked it at 20 out of curiosity; the frets are playable and properly positioned. I only occasionally go up there doing scales because it seems silly to practice what I don't really play. I do sometimes play open chords an octave up on the fretted strings.
"First you master your instrument, then you master the music, then you forget about all that ... and just play"
Charlie "Bird" Parker